Why is this water bubbling up out of the ground? Have you ever seen anything like it? Groundwater, which is stored in rocks below the surface, can flood the landscape after a severe storm when the ground is saturated. It can also surface in low-lying areas.

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Launch Lab: How solid is Earth’s surface?

It feels solid, but just how solid is “solid ground”? Believe it or not, the soil and rock beneath your feet are not entirely solid.

Hutchings Photography/Digital Light Source

1.Read and complete a lab safety form.

2.Fill a large, empty jarwith golf balls.

3.Follow the instructions given by your teacher.

Think About This

1.At what point did you think the jar was full?

2.Do you think the particle size of the soil affects how quickly water can move through it?

3.Key ConceptDoes water flow more easily through sediments of equal size or sediments with a variety of different sizes?

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Groundwater

Some water that falls to Earth as precipitation soaks into the ground. Generally, water that lies below ground is calledgroundwater.Water seeps through soil and into tiny pores, or spaces, between sediment and rock. If you have ever been inside a cave and seen water dripping down the sides, you’ve seen groundwater seeping through rock.

1.Key Concept CheckWhat is groundwater?

In some areas, groundwater is very close to the surface and keeps the soil wet. In other areas, especially deserts and other dry climates, groundwater is hundreds of meters below the surface.

Groundwater can remain underground for long periods of time—thousands or millions of years. Eventually, it returns to the surface and reenters the water cycle. Humans interfere with this process, however, when they drill wells into the ground to remove water for everyday use.

Importance of Groundwater

The water beneath Earth’s surface is much more plentiful than the freshwater in lakes and streams. Recall that groundwater is about one-third of Earth’s freshwater. Groundwater is an important source of water for many streams, lakes, and wetlands. Some plant species absorb groundwater through long roots that grow deep underground.

People in many areas of the world rely on groundwater for their water supply. In the United States, about 20 percent of the water people use daily comes from groundwater.

The Water Table

As illustrated in Figure 1,groundwater seeps into tiny cracks and pores within rocks and sediment. Near Earth’s surface, the pores contain a mixture of air and water. This region is called the unsaturated zone. It is called unsaturated because the pores are not completely filled with water. Farther beneath the surface, the pores are completely filled with water. This region is called the saturated zone. The upper limit of the saturated zone is called thewater table.

2.Reading CheckWhat is the water table?

PorosityRocks vary in the amount of water they can hold and the speed with which water flows through the rock. Some rocks can hold a lot of water and some rocks cannot. Porosityis a measure of rock’s ability to hold water. Porosity increases with the number of pores in the rock. The higher the porosity, the more water a rock can contain.

PermeabilityThe measure of water’s ability to flow through rock and sediment is calledpermeability. This ability to flow through rock and sediment depends on pore size and the connections between the pores. Even if pore space is abundant in a rock, the pores must form connected pathways for water to flow easily through the rock.

Figure 1Aquifers store ground water. Groundwater becomes surface water, including springs and streams, in places where the water table meets the surface of the land.

Groundwater Flow

Just as runoff flows downhill across Earth’s surface, groundwater flows downhill beneath Earth’s surface. Groundwater flows from higher elevations to lower elevations. In low-lying areas at Earth’s surface, groundwater might eventually seep out of the ground and into a stream, a lake, or a wetland, as also shown in Figure 1.In this way, groundwater can become surface water. Likewise, surface water can seep into the ground and become groundwater. This is how groundwater is replenished.

WellsPeople often bring groundwater to Earth’s surface by drilling wells like the one shown in Figure 1.Wells are usually drilled into an aquifer—an area of permeable sediment or rock that holds significant amounts of water. Groundwater then flows into the well from the aquifer and is pumped to the surface.

Precipitation helps replace groundwater drawn out of wells. During a drought, less groundwater is replaced, so the water level in a well drops. The same thing happens if water is removed from a well faster than it is replaced. If the water level drops too low, a well runs dry.

SpringsA spring forms where the water table rises to Earth’s surface, as shown in the lesson opener. Some springs bubble to Earth’s surface only after heavy rain or snowmelt. Many springs fed by large aquifers flow continuously.

Human Impact on Groundwater

If polluted surface water seeps into the ground, it can pollute the groundwater below it. Pollutants include pesticides, fertilizers, sewage, industrial waste, and salt used to melt ice on highways. Pollutants can travel through the ground and into aquifers that supply wells. People’s health can be harmed if they drink contaminated water from a well.

The water in an aquifer helps to support the rocks and soil above it. In some parts of the world, water is being removed from aquifers faster than it can be replaced. This creates empty space underground. The empty space underground cannot support the weight of the overlying rock and soil. Sinkholes form where the ground collapses due to lack of sufficient support from below.

3.Key Concept CheckHow do human activities affect groundwater?

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Wetlands

Water often collects in flat areas or depressions that are too shallow to form lakes. Conditions like these can create a wetland—an area of land that is saturated with water for part or all of the year. Wetlands also form in areas kept moist by springs, and in areas along the shores of streams, lakes, and oceans. The water in a wetland can remain still or flow very slowly.

Types of Wetlands

Scientists identify wetlands by the characteristics of the water and soil and by the kinds of plants that live there. There are three major types of wetlands, as shown in Table 1.Bogs form in cool, wet climates. They produce a thick layer of peat—the partially decayed remains of sphagnum moss. Peat holds water, so bogs rarely dry out. Unlike bogs, marshes and swamps form in warmer, drier climates and do not produce peat. Marshes and swamps are supplied by precipitation and runoff. They can temporarily dry out in hot, dry weather.

Wetlands provide important habitat for plants and wildlife. They help control flooding and erosion and also help filter sediments and pollutants from water.

HabitatA wide variety of plants and animals live in wetlands, as shown in Figure 2.Wetlands provide plentiful food and shelter for young and newly hatched animals, including fish, amphibians, and birds. Wetlands are also important rest stops and food sources for migrating animals, especially birds.

Flood ControlWetlands help reduce flooding because they store large quantities of water. They fill with water during the wet season and release the water slowly during times of drought.

Erosion ControlCoastal wetlands help prevent beach erosion. Wetlands can reduce the energy of wave action and storm surges—water pushed onto the shore by strong winds produced by severe storms.

FiltrationWetlands help keep sediments and pollutants from reaching streams, lakes, groundwater, or the ocean. They are natural filtration systems. Runoff that enters a wetland often contains excess nitrogen from fertilizers or animal waste. Plants and the bacteria in wetland soils absorb excess nitrogen. Wetland plants and soils also trap sediments and help remove toxic metals and other pollutants from the water.

1.Key Concept CheckWhy are wetlands important?

Frank Krahmer/Masterfile

Figure 2Wetlands are an important habitat for wildlife, providing water, food, and shelter.

Human Impact on Wetlands

Many wetlands throughout the world have been drained and filled with soil for roads, buildings, airports, and other uses. The disappearance of wetlands has also been associated with rising sea level, coastal erosion, and the introduction of species that are not naturally found in wetlands. Scientists estimate that more than half of all wetlands in the United States have been destroyed over the past 300 years.

The Louisiana coastline has lost about 310 km2of wetlands since 1950. Figure 3shows some of the changes that have taken place on the Louisiana coast. A lack of coastal wetlands may have contributed to widespread flooding that occurred when Hurricane Katrina struck the area in 2005.

2.Key Concept CheckHow do human activities affect wetlands?

Kevin Fleming/CORBIS

Figure 3Large portions of the coastal wetlands of Louisiana have been filled in to build roads and buildings. Scientists and engineers have proposed that future flooding could be avoided by restoring some wetland areas.

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WORDORIGIN

sphagnumfrom Greek sphagnos, means “a spiny shrub”

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MiniLab: Can you model freshwater environments?

Sources of groundwater and surface water are similar. When the ground becomes saturated with water following a storm, the excess water can flow into lakes or streams. Groundwater may also surface and create wetlands.

1.Read and complete a lab safety form.

2.Half fill a plastic shoeboxwith sand.Flatten out the surface of the sand.

3.While watching the side of the box as you pour, add enough water so the water level, or water table, is even with the top of the sand.

4.Scoop some sand from one end of the box and pile the excess sand on the other end of the box to form highlands. Create an area between the scooped hole and the pile where the level of the sand is at the water table.

5.View the plastic shoebox from the side. Draw a cross section of your model in your Science Journal. Include the water table on your cross section.

6.Add a few drops of food coloringto the highlands to represent pollution. Using a paper cupwith holes in the bottom, model precipitation by dripping water down over your model. Observe.

Hutchings Photography/Digital Light Source

Analyze and Conclude

1.Labelthe lake, wetlands, highlands and water table on your diagram and identify where the water table is above, at the same level as, and below the surface.

2.Describethe path the “pollution” follows.

3.Key ConceptHow does this lab demonstrate the behavior of groundwater?

Lesson Review

Visual Summary

Groundwater fills pores in soil and rocks in the saturated zone. The water table marks the top of the saturated zone.

Jason Edwards/Getty Images

A wetland is an area of land that is saturated with water for part or all of the year.

Brand X Pictures/PunchStock

Human involvement in alteration and destruction of wetlands can have devastating effects.

Frank Krahmer/Masterfile

What do you think NOW?

You first read the statements below at the beginning of the lesson.

5.People use groundwater as a source of water.

6.Wetlands can naturally filter pollutants from groundwater.

Did you change your mind about whether you agree or disagree with the statements? Rewrite any false statements to make them true.

Lesson Assessment

Use Vocabulary

1.Distinguishbetween porosityand permeability.

2.Use the termgroundwaterin a complete sentence.

3.Definewater tablein your own words.

Understand Key Concepts

4.Where is the saturated zone located?

A.above the water table

B.below the water table

C.beside the water table

D.within the water table

5.Explainhow groundwater can become surface water.

6.Explainhow groundwater that is used as drinking water reaches homes.

7.Listthree reasons wetlands are important to people and the environment.

8.Which is NOT a reason wetlands are important?

A.affect climate

B.control erosion

C.filter out pollutants

D.provide wildlife habitat

9.Which type of wetland is shown in the photo below?

Panoramic Images/Getty Images

A.bog

B.fen

C.marsh

D.swamp

10.How does a well run dry?

A.Groundwater is removed more slowly than it is replaced.

B.Groundwater is removed more quickly than it is replaced.

C.Groundwater gets trapped in an aquifer.

D.Groundwater becomes contaminated with pollutants.

Interpret Graphics

11.Organize InformationCopy and fill in the graphic organizer below to describe the different types of wetlands.

Critical Thinking

12.Design an experimentto test whether groundwater from a well has been contaminated by wastewater from a nearby sewage treatment plant.

13.ExplainA local developer wants to drain a wetland in your city and build a shopping mall in its place. Write a letter to the editor of your local newspaper to explain the possible outcome of the development.

14.Inferwhy wetlands are good habitats for newly hatched and juvenile animals, such as fish and amphibians.

15.Explainhow humans affect wetlands.

16.Compare and contrastan aquifer and a spring.

17.Design a modelthat would help you explain the difference between porosity and permeability to a group of elementary school students.

18.Where is Earth’s freshwater? Describe its general form, such as ice or liquid water. Also describe its general location, such as above ground, below ground, etc. Approximately what percentage of Earth’s freshwater is at these locations?